Superheating steam generator

ABSTRACT

Described herein is a shell and tube vapor generator in which vaporizable liquid is evaporated and superheated within the same vessel. Downflow and upflow passages are provided for conducting the vaporizable liquid in heat exchange relation with the heating medium that is conducted through the tubes. The downflow passages into which subcooled feedwater is introduced are defined by a plurality of mutually spaced conduits disposed between the shell wall and the tube bundle. The arrangement permits the shell wall to be wetted by liquid at substantially saturated temperature throughout the evaporation region of the chamber thus to reduce the stresses within the tube material caused by thermal loading.

United States Patent Zmola [54] SUPERHEATING STEAM GENERATOR [72] Inventor: Paul C. Zmola, Bloomfield, Conn.

[73] Assignee: Combustion Engineering, Inc.,Wind-- sor, Conn.

[22] Filed: Nov. 20, 1970 [21] Appl. No.: 91,352

[52] US. Cl...... ..l22/32, l22/34 [Sl] Int. Cl ..F22b 1/06 [58] Field of Search ..l22/32, 34

[56] References Cited UNITED STATES PATENTS 3,447,509 6/ I969 Sprague ..l22/32 FOREIGN PATENTS OR APPLICATIONS 1,086,243 10/1967 Great Britain I 22/34 [15] 3,683,866 [451 Aug. 15,1972

v Kessler [57] ABSTRACT Described herein is a shell and tube vapor generator in which vaporizable liquid is evaporated and superheated within the same vessel. Downflow and upflow passages are provided for conducting the vaporizable liquid in heat exchange relation with the heating medium that is conducted through the tubes. The downflow passages into which subcooled feedwater is introduced are defined by a plurality of mutually spaced conduits disposed between the shell wall and the tube bundle. The arrangement permits the shell wall to be wetted by liquid at substantially saturated temperature throughout the evaporation region of the chamber thus to reduce the stresses within the tube material caused by thermal loading.

4 Claims, 2 Drawing Figures PATENTEDwa 15 m2 3.683.866

sum 1 or 2 INVENTOR PAUL C. ZMOLA F/G-l BY ATTOE/VfY PATENTEDAuc 15 m2 SHEET 2 0F 2 INVENTOR. PAUL C. ZMOLA SUPERHEATING STEAM GENERATOR BACKGROUND OF THE INVENTION Shell and tube vapor generators are known in which evaporation and superheating of vaporizable liquid occur within the same vessel. Conventionally, such apparatus comprise a plurality of straight tubes arranged in a tube bundle and housed within the vessel, the interior of which is divided by means of an annular baffle into an annular downflow passage and an axial upflow passage. The tube bundle is disposed within the upflow passage while the downflow passage is devoid of tubes. Relatively cool vaporizable liquid is admitted to the downflow passage, is preheated and normally enters the upflow passage at about saturation temperature. Evaporization and superheating of the liquid occurs as it passes along the upflow passage in heat exchange relation with the heating medium that is conducted by the tubes. Fluid circulation of the vaporizable liquid is established through the vessel by the thermal siphonic action created as a result of the difference in density between the cooler fluid in the downflow passage and the higher temperature fluid in the upflow passage.

Problems are encountered in the operation of such apparatus due to the difference in metal temperatures that exist between the shell and the tubes. The latter are thin walled members and are subjected to higher average metal temperatures since they conduct the higher temperature heating fluid. The shell, on the other hand, contains a greater mass of metal andis maintained at a lower average metal temperature since they conduct the higher temperature heating fluid. The shell, on the other hand, contains a greater mass of metal and is maintained at a lower average metal temperature since its wall is wetted by the cooler downflow liquid. In fact, in the region of the feedwater inlet, the

shell wall normally comes in contact with liquid thatmay be as much as 150 cooler than that conducted by the tubes. As a result of the induced differential thermal expansion between the tubes and the shell, the former are subjected to stresses which, if not protected against, could reach dangerous proportions resulting in partially weakening or failure of the tubes by buckling.

Heretofore, it has been contemplated to guard against these thermal stresses either by providing expansion joints in the shell, incorporating expansion loops in each of the tubes, or by restricting the operating range of the unit. The first two of the abovemen tioned alternatives considerably increase the fabrica tion costs of such apparatus. They also create a need for additional maintenance thus increasing operating expenses of the apparatus. The last mentioned alternative restricts the utility of the apparatus.

It is to an improved solution to this problem, therefore, that the present invention is directed.

SUMMARY OF THE INVENTION According to the invention means are provided to increase the average metal temperature of the shell along its length thereby to reduce the differential temperature between the shell and the heat exchange tubes and, concomitantly, the thermal stresses developed in the tubes.

Stated in its simplest terms, the present invention contemplates replacing the annular baffle that defines the downflow passage in an otherwise substantially conventional vapor generating apparatus of the disclosed type with a plurality of conduits adapted to conduct the downflow liquid. The conduits are disposed between the shell and the tube bundle in circumferentially spaced relation thereby permitting the shell wall to be wetted by higher temperature liquid as compared with similar apparatus of the earlier design. By means of the disclosed arrangement, the shell is exposed to fluid at substantially saturated temperature throughout the entire region that was formerly exposed to subcooled liquid at temperatures varying between that of the cold entering liquid and those approaching saturation temperature at the juncture of the downflow and upflow passages in similar prior art apparatus.

For a better understanding of the invention, its operating advantages and the specific objects obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred embodiment of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation of a vapor generator embodying the present invention, and

FIG. 2 is a section taken along line 2-2 of FIG. 1.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 depicts, somewhat schematically, a vapor generator 10 of the shell and tube type constructed according to the present invention. It comprises a vertically elongated cylindricalpressure vessel 12 that is closed at its opposite ends by upper and lower domed closure member 14 and 16, respectively. The interior of the shell is provided with a pair of axially spaced,

transversely extending tube sheets, 18 and 20, which integrally attach to the shell wall and which divide its interior into three fluidly distinct chambers, indicated as heating fluid inlet and outlet chambers, 22 and 24, and vapor generating chamber 26. A plurality of Iongitudinally extending, straight heat exchange tubes 28, arranged in a tube bundle, extend through the vapor generating chamber 26 and have their opposi t ends attached to the respective tube sheets 18 and 20 in fluid communication with the heating fluid inlet and outlet chambers 22 and 24. A plurality of lattice type tube supports 25 are spaced along the length of the tube bundle to support the tubes 28 and to prevent excessive tube vibration during operation of the unit. Nozzles 30 and 32 communicate with the chambers 22 and 24 respectively for circulating heating fluid between the vapor generator 10 and a source not shown).

The wall of the pressure shell 12 is penetrated by a feedwater inlet nozzle 34 that connects the apparatus with a source of vaporizable liquid. The nozzle 34 connects with an annular manifold 36, or other suitable arrangement for distributing the feedwater flow, that is disposed within the interior of the shell between the shell wall and the tube bundle. The manifold 36 is provided with a plurality of circumferentially spaced, depending injector nozzles 38 that discharge feed liquid intothe chamber 26 to form a body of liquid therein, such that, at full load operation, the vaporizable fluid at .the level indicated as 40 in FIG. 1 ia at a high quality,

for example percent or more, while at low load operation substantially dry vapor exists at this level.

For purpose of discussion, the level 40 can be considered as dividing the chamber 26 into a lower evaporation portion 42 and an upper vapor heating region 44.

The described apparatus further includes baffle structure, including transverse plates 46 and annular baffle 48, that is provided in the vapor heating portion 44 of the unit for conducting the evaporated fluid through this region of the chamber 26 to a vapor discharge nozzle 50 from whence superheated vapor is conducted to a point of use. Plates 46 are arranged in mutually spaced relation, as shown, and serve to conduct the generated vapor along a tortuous path across the tubes 28 in heat exchange relation with the heating medium conducted thereby. Annular baffle 48 is spaced from the shell wall in the upper region of the vessel and defines a concentric passage52 with which the vapor discharge nozzle 50 communicates.

According to the present invention a plurality of elongated, open ended cylindrical conduits 54 are. disposed in the annular space 56 defined between the interior wall of shell 12 and the periphery of the bundle of tubes 28. The conduits 54 are located on circumferential spacing with their external surface spaced away from the surface of the shell wall so as to fully expose the latter to the interior region of the chamber 26. The conduits extend parallel to the axis of the shell from a level subjacent the manifold 36 to one slightly above the upper surface of, the lower tube sheet 20 whereby their interior is placed in fluid communication with the lower end of the chamber 26. At its upper end each conduit is adapted to receive the discharge end of an injector nozzle 38 by means of which relatively cool feed liquid is admitted to the downflow passage defined by the respective conduits for ultimate discharge into the lower region of the vapor generation chamber 26.

The respective diameters of the conduits 54 and the nozzles 38 are such as to space the nozzle from the interior wall of the conduit. By means of this arrangement, an opening is provided about the conduit end whereby heated vaporizable fluid of high quality or as dry vapor is admitted to the downflow passage from the vapor generation chamber 26 to heat the entering feed liquid, principally by condensation.

The operation of the herein described vapor generating apparatus is as follows. High temperature heating fluid enters the inlet chamber 22 through nozzle 30 and is conducted by tubes 28 through the chamber 26 to be discharged into outlet chamber 24 from whence it is removed through outlet nozzle 32. Low temperature feed liquid enters the apparatus through nozzle 34 and passes into the annular manifold 36 from whence it is discharged through nozzles 38 into the interior of the downflow conduits 54. The discharge of the feed liquid is regulated to maintain a body of liquid with the vapor generating chamber 26 such that the pressure within chamber 26 is maintained constant or suitably regu lated with respect to load.

vaporizable fluid of high quality or as substantially dry vapor is coincidentally drawn into the upper ends of the conduits to mix with the incoming feedwater. In contacting the relatively cool feedwater the vapor is condensed thereby to raise the temperature of the feedwater within the conduits 54. Due to the difference in density between the cooler fluid contained in conduits 54 as compared with that immersing the tubes 28 within the interior of the chamber 26, fluid circulation is established in the unit with the resulting flow being downwardly through the conduits and upwardly through the vapor generating chamber. In passing through the vapor generating chamber, the feed liquid is first evaporated in its lower portion 42 with the resulting vapor being passed in heat exchange relation with the exposed surface of the tubes 28 in the upper region 44 thereby to impart a slight amount of superheat to the vapor prior to its admission to the annular passage 52 and ultimate discharge through nozzle 50.

Because the downflow passage in the present arrangement is formed by independent conduits 54 disposed in mutually spaced relation about the annular space 56, the wall of the shell 12 is caused to be wetted by the liquid contained in the lower region 42 of the chamber 26. The fact that this liquid is at, or substantially at, saturated temperature results in the shell wall being exposed to substantial uniform temperatures from its lower end adjacent the tube sheet 20 to the region just above the feed inlet nozzle 34. This wetting of the shell by liquid at saturaturated temperature results in increasing the average metal temperature of the shell as compared with arrangements wherein the shell is caused to contact subcooled liquid thereby to reduce the temperature differential existing between the tubes 28 and the shell, and concomitantily to lessen the thermal stresses that would be established within the tubes Above the bottom of the annular passage 52, the shell wall may be exposed to a slightly higher temperature due to the fact that it is contacted by superheated vapor. This is not detrimental to the operation of the apparatus and, in fact, will even improve its stress relieving characteristics in that the average metal temperature of the shell is further increased.

Furthermore, the present invention eliminates the danger of chilling the vessel shell due to flooding the downcomer passages as could conceivably occur when the pressure drop experienced by the vaporizable fluid in flowing through the chamber 26 exceeded design limits. Should this condition occur in apparatus incorporating the present invention the cool liquid discharged from the upper end of the conduits will simply mix with the heated fluid surrounding the conduits and wetting the interior shell surface thereby to prevent excessive thermal stresses from being generated in the shell.

While a preferred embodiment of the invention has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the present invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not by limitation.

What is claimed is:

1. Vapor generating apparatus including:

a. an elongated shell defining a substantially closed pressure vessel containing a pair of axially spaced, transversely extending tube sheets integrally joined to the wall of said shell and defining a vapor generating chamber therebetween;

b. a plurality of substantially straight heat exchange tubes disposed in said vapor generating chamber with their opposite ends attached to the respective 2. Apparatus as recited in claim 1 wherein said contube sheets, said tubes being arranged in a bundle, duits are open at their upper end and include means for the outer periphery of which is concentrically in vapor generated in said vapor generation Spaced from the Wall of Said She; I chamber into said conduits in mixed relation with the c. means for circulating heating fluid through said 5 f d ater d i d h t tubes; and 3. Apparatus as recited in claim 2 wherein said feedmeans for Circulating Vaporizable liquid through water passing means includes an injector nozzle ex- Said vapor generating chamber in heat exchange tending into the upper end of each of said conduits in relation with Said tubes comprising: spaced relation from the wall thereof. feedwatef inlet nozzle means Penetrating the 10 4. Apparatus as recited in claim. 2 wherein said feed- Wall of Sald She, water passing means includes:

3 ahty of l g g' space; condsl.ts a. an annular manifold surrounding said tube bundle e mmg Separa 6 Own Ow passages lspose m within said vessel and communicating with said the space between the outer periphery of the tube bundle and the wall of said shell, the spaces 15 feedwater mlei nozzle and between said conduits placing the wall of said l f f shell in open fluid communication with said nozzles depending from the ower Sur ace 0 Sal vapor generation Chamber al on g substantially manifold for discharging feedwater therefrom, said the full length of Said downflow passage nozzles extending into the upper ends of said coniii. means for passing feedwater from said inlet duts m spaced relatlon from the wall thereof nozzle means to the respective conduits.

b. a plurality of circumferentially spaced injector 

1. Vapor generating apparatus including: a. an elongated shell defining a substantially closed pressure vessel containing a pair of axially spaced, transversely extending tube sheets integrally joined to the wall of said shell and defining a vapor generating chamber therebetween; b. a plurality of substantially straight heat exchange tubes disposed in said vapor generating chamber with their opposite ends attached to the respective tube sheets, said tubes being arranged in a bundle, the outer periphery of which is concentrically spaced from the wall of said shell; c. means for circulating heating fluid through said tubes; and d. means for circulating vaporizable liquid through said vapor generating chamber in heat exchange relation with said tubes comprising: i. feedwater inlet nozzle means penetrating the wall of said shell, ii. a plurality of circumferentially spaced conduits defining separate downflow passages disposed in the space between the outer periphery of the tube bundle and the wall of said shell, the spaces between said conduits placing the wall of said shell in open fluid communication with said vapor generation chamber along substantially the full length of said downflow passage, iii. means for passing feedwater from said inlet nozzle means to the respective conduits.
 2. Apparatus as recited in claim 1 wherein said conduits are open at their upper end and include means for passing vapor generated in said vapor generation chamber into said conduits in mixed relation with the feedwater admitted thereto.
 3. Apparatus as recited in claim 2 wherein said feedwater passing means includes an injector nozzle extending into the upper end of each of said conduits in spaced relation from the wall thereof.
 4. Apparatus as recited in claim 2 wherein said feedwater passing means includes: a. an annular manifold surrounding said tube bundle within said vessel and communIcating with said feedwater inlet nozzle, and b. a plurality of circumferentially spaced injector nozzles depending from the lower surface of said manifold for discharging feedwater therefrom, said nozzles extending into the upper ends of said conduits in spaced relation from the wall thereof. 